Different types of programmable data processors for telephone exchanges are known from the literature including various patent specifications such as German Pat. Nos. 1,914,578 (Siemens), 1,549,550 (Ericsson), and 1,474,097 (Western Electric). Owing to economic considerations these known processors are used mostly in larger switching systems with several thousand subscriber lines or with a load equivalent to such a number of lines. The use of controlled switching based on stored programs in smaller exchanges is still a matter in dispute, mainly on account of the high costs of the control system itself.
Programmable data processors comprise generally a memory, means for storing and transferring data for addressing the memory, a set of working registers including an input-output coupler storing the internal operational data of the processor and establishing a connection towards the peripheral units, and an instruction-executing component including typically an instruction register, a (mostly microprogrammed) control block and an arithmetical and logic unit.
The memory serves for storing the programs required for the operation of the controlled switching unit, along with data including those relating to the operational states of the lines and of the circuits of an associated interface unit. The required capacity of the memory for storing the program and the various data is in first approximation identical in case of all larger centers (see for instance the systems AKE or AXE of the Company Ericsson). In smaller switching systems the amount of data to be processed decreases substantially proportionally to the size of the exchange; however, the number of programs defining the controlling algorithms can decrease only if the associated functions also become simpler. The basic functions in telephone technology are identical in larger and smaller exchanges. Thus, for switching units of smaller size the memory capacity is defined essentially by the size of the program store. Given a predetermined set of algorithms and programmable ability, the size of that store depends mostly on the machine-coded programming language which is determined by the structural design of the processor. Thus, an improved design of the processor may significantly reduce the size of the program store.